Analysis of structure-localized mercury in Ottawa river sediments by scanning electron microscopy/energy-dispersive x-ray microanalysis technique

Abstract

Sediment samples of Ottawa River were examined using a relatively new technique, electron microscopy/energy dispersive (EM/ED) microanalyzer, to identify and document the binding sites for mercury on sediments and biota. Samples were deposited on a Nucleopore filter, dried, and then mounted on an aluminum stub for EM/ED analysis. Better images for documentation were obtained by depositing the samples directly on a carbon planchet, thus eliminating the insulation from the filter and the x-ray emmission from the aluminum stub. The salient features of ED x-ray microanalyzer, such as identification of most major and minor elements in a single analysis, and simple spectrum of x-ray energies versus atomic number of elements, are listed. Pitfalls in the methodology of the positive identification of elements resulting from peak shifting and peak overlap in the complex spectra of environmental samples are discussed. Remedies for successful resolution of broad spectra into individual elemental peaks are clearly outlined. Analyses of sediment samples revealed the presence of a variety of structures in aquatic environments with varying elemental compositions. Data gathered on mercury-bearing structures in the present study showed that mercury binding is on sulfur sites but never in a stoichiometric ratio; not all sulfur sites accumulated mercury. Possible reasons for sulfur sites void of mercury are (1) uneven distribution of binding sites, (2) concentrations of mercury too low to saturate all the binding sites, or (3) possible desorption of mercury from the binding sites by either chemical or biologic processes.